The ability to gather statistical information over large populations of cells using flow cytometry of cells labeled with fluorescent probes has contributed tremendously to the diagnostics and study of cellular function. In a flow cytometer or a fluorescence-activated cell sorter (FACS), cells are interrogated by flowing them past a detector in a continuous stream of buffer and sorted by molecules either confined within the cells or bound to the cell surface, rather than by properties of the molecules the cells secrete (Köster et al., Lab on a Chip 8, 1110-1115 (2008); Herzenberg et al., Clin. Chem. 48, 1819-1827 (2002); Carroll and Al-Rubeai, Expert Opin. Biol. Ther. 4:1821-1829 (2004)). Low expression of a target molecule can lead to an insufficient or low-intensity detection signal. Further, to sort cells based on secreted molecules, additional tests are usually performed by enzyme-linked immunosorbent assay (ELISA) or comparable methods to verify secretion of a particular molecule of interest from a population of cells rather than from an individual cell. It is also challenging to determine time-dependent variability of secretion from single cells by current methods.